Not applicable.
Not applicable.
This invention relates to glossy paint formulations containing blends of latexes of hard and soft polymers of vinyl acetate. In another aspect it relates to paints capable of forming coatings having improved resistance to blocking without diminished gloss. In still another aspect it relates to a method of making non-blocking semi-gloss paint.
The need today for architectural coating materials free from volatile organic content (VOC) for both safety and health reasons is well documented. The switch to water-borne or high solids paints has answered this need, but there still remains a problem with respect to paints where surface luster is important, such as in satin, semi-gloss and high gloss paints. Such paints need to have good blocking resistance to keep painted surfaces, such as on doors and windows, from sticking. This problem has been addressed by incorporating organic solvents, but this approach partially defeats a major advantage of using water-based paints. While the blocking problem can be alleviated by incorporating xe2x80x9chardxe2x80x9d polymers, i.e. polymers having a relatively high glass transition temperature (Tg), into the formulation, this step has generally resulted in the reduction of gloss values in the finished coating.
Daniels in xe2x80x9cVinyl Acetate Polymersxe2x80x9d, Encyclopedia of Polymer Science and Engineering, Vol. 17, p. 393-425 (1989) states that it has long been the practice to modify latex film properties by blending one polymer latex with another, for example, making a xe2x80x9csoftxe2x80x9d polymer less blocking with a xe2x80x9chardxe2x80x9d polymer. As examples, he describes compatible solvent blends of poly(vinyl acetate) with poly(ethyl methacrylate), cholesterol and derivatives thereof, poly(butyl acrylate), poly(ethylene oxide) with poly(methyl methacrylate), and terpolymers of acrylonitrile, methyl acrylate and Na methylpropenyl sulfonate. The issue of gloss is not addressed.
Snyder, U.S. Pat. No. 5,308,890 (1994) discloses an approach to the blocking problem in multi-stage latex polymers by incorporating into the multi-stage latex polymer a latex which is not film-forming at ambient temperature. The nonfilm-forming polymers named are copolymers of butyl acrylate or ethyl acrylate with methyl methacrylate and methacrylic acid and/or styrene (with Tg (glass transition temperature) values above room temperature). Film gloss was not an issue. Never-the-less, it was observed by Patel et al. in xe2x80x9cCharacterization of Latex Blend Films by Atomic Force Microscopyxe2x80x9d, Polymer, Vol. 37, No. 25, pp. 5577-82 (1996) that the incorporation of a latex of a hard polymer into the latex of a soft polymer increases surface roughness of the polymer film in proportion to the hard polymer in the blend. The polymers studied were copolymers of butyl acrylate and methyl methacrylate. A similar observation was made by Fream and Magnet in xe2x80x9cLow VOC, High Performance Coating Formulationxe2x80x9d, Farg och Lack Scandinavia, 1/1998, pp. 4-9. These authors discussed the effects of blending two carboxylated styrene acrylate latexes, one having a Tg of 57xc2x0 C. and the other a Tg of 0xc2x0 C., in coating formulations. Although a reduction in film surface tack was noted, there was also considerable reduction in gloss.
Friel, U.S. Pat. No. 5,731,377 (1998) discloses paints based on emulsion polymer blends of two different polymers having relatively high and low Tg values, e.g. one polymer having a Tg above 20xc2x0 C. and the other a Tg below 15xc2x0 C. The purpose given for making such blends is to improve blocking resistance. Although an extensive list of possible monomers is given, the only polymers described as suitable for both the high and low Tg polymers are copolymers of butyl acrylate and styrene and/or methacrylic acid and/or methyl methacrylate, and copolymers of ethylhexyl acrylate, styrene and acrylonitrile for high Tg polymers. No data or evaluations are given on the effect of high Tg polymers on paint gloss. It is clear from the foregoing references that the problem of eliminating VOC from paint formulations while improving blocking resistance without loss of gloss in the paint surface has heretofore not been solved.
We have now discovered that a water-borne glossy paint can be made from a blend of latexes of vinyl acetate polymer and vinyl acetate copolymer having relatively high and low Tg values, respectively, both polymers being film-forming at room temperature, with marked improvement in blocking resistance but without significant reduction of gloss. In many cases gloss is actually improved which is very surprising in view of the prior art.
The vinyl acetate polymer is preferably a homopolymer but can contain up to 15 weight percent of comonomer such as other vinyl esters and acrylates. This vinyl acetate polymer must have a dry glass transition temperature (Tg) above room temperature (23xc2x0 C.) but a wet Tg below room temperature. This polymer must also be film forming at room temperature. These vinyl acetate polymers are unique in this respect and differ functionally as well as chemically from the high Tg (xe2x80x9chardxe2x80x9d) polymers used in the prior art to enhance blocking resistance in glossy paints containing soft polymers.
The copolymer of vinyl acetate can be a copolymer of vinyl acetate and ethylene or a copolymer of vinyl acetate and an alkyl acrylate, the alkyl group preferably containing 1 to 8 carbons. The copolymer is also film forming at room temperature, having a Tg less than 20xc2x0 C. The proportion of high Tg polymer of vinyl acetate in the blend with the low Tg copolymer of vinyl acetate is generally in the range of 5 to 75 weight percent, preferably 10 to 60 weight percent, based on the combined polymer solids. Latex blends in which from 20 to 50 weight percent of the polymer solids are from the high Tg polymer of vinyl acetate are demonstrated in the data of the examples herein and are still more preferred.
This invention, which is embodied in both composition of matter and the process steps taken to form the composition, has application only to paints in which gloss or luster is important. These paint applications include satin and semi-gloss paints as well as high gloss paints. For satin paints, the ASTM gloss (60xc2x0) gloss value will be above 10. For semi-gloss paints, the gloss value will be above 30. The preferred systems will be semi-gloss paints, however, the compositions of this invention can also be employed for satin as well as high gloss applications. Glossy in the invention refers to paint surfaces having gloss values in the range of satin, semi-gloss and high gloss applications. Flat paints would have gloss values (ASTM 60xc2x0 gloss) less than 10 and are outside the scope of this invention.
The large market for architectural coatings (paint) has been continually undergoing changes in product type and formulation due to increasing need and regulation for lower VOC (volatile organic content) products. Initially (and continuing), the decrease in VOC paints involved a switch from solvent based to water-borne or high solids paints. The limitation on high solids due to viscosity problems has resulted in even more effort being expended on the development of water-borne paints. Nevertheless, for water-borne paints where gloss is important (e.g. satin, semi-gloss, high gloss) VOC addition is still necessary because the desired combination of properties (especially blocking resistance) has not been achievable without VOC addition. In water-borne paint, it has been well-recognized that a combination of relatively low Tg and high Tg polymer emulsions are necessary to yield film formation and blocking resistance at low to no VOC. This combination, however, leads to decreased gloss and thus is only viable in flat (low gloss) paints. With flat paints, the higher filler loading improves blocking resistance and gloss is not an issue. The present invention applies to glossy paints and uses a specific combination of low Tg/high Tg vinyl acetate polymer emulsions to provide improved blocking resistance without significantly diminishing gloss values. Generally specific formulations are utilized for various levels of gloss desired in paints. These formulations can be for satin, semi-gloss and high gloss as well as flat applications. The level of pigment/filler are varied to yield the desired properties. It is desired to modify the low Tg emulsion polymer with a high Tg emulsion polymer whereby the blocking resistance improves but the gloss is basically unchanged or improved. If the gloss is significantly decreased, changes in the formulation will be required and may result in loss of other key properties such as scrub resistance.
Certain of the prior art references (e.g. Freil) note high Tg/low Tg polymer combinations for paint applications and even semi-gloss applications, but fail to note the negative effects on gloss by adding high Tg nonfilm-forming polymer emulsions to low Tg polymer emulsions. The polymers discussed and investigated include acrylics and styrenics. Poly(vinyl acetate) offers unique differences compared to acrylics and styrenics relative to the specific application area of this invention. The lower water sorption of acrylics and styrenics will not allow for room temperature film formation with Tg""s above room temperature. Poly(vinyl acetate), on the other hand, with a Tg of 33-35xc2x0 C. (determined calorimetrically) is film-forming at room temperature when applied as an aqueous emulsion.
This invention offers an approach to low VOC, water-borne satin, semi-gloss or even high gloss paints where the combination of properties required for these applications are difficult to meet with a single emulsion polymer. It has been found that a specific combination of emulsion polymers (in the form of a simple emulsion blend) yields better properties than the unblended constituents for the architectural coating applications noted above. This invention involves the addition of a water-borne polymer of vinyl acetate, preferably poly(vinyl acetate) (PVAc), to a lower Tg water-borne vinyl acetate copolymer. Lower Tg as used herein means a glass transition temperature measured on dry samples calorimetrically (mid-point) at 10xc2x0 C./min. heating rate of below room temperature (defined as 23xc2x0 C. for this disclosure). The Tg of the poly(vinyl acetate) measured on dry samples by the above noted protocol is above room temperature.
The polymer of vinyl acetate of this invention can be a homopolymer, i.e. poly(vinyl acetate), or a copolymer containing at least 85 weight percent vinyl acetate such that the dry Tg is above room temperature and the wet (water immersion) Tg is below room temperature. Comonomers which can comprise up to 15 weight percent of this vinyl acetate polymer include other vinyl esters such as vinyl propionate, vinyl versatate and the like, methacrylates such as methyl methacrylate, ethyl methacrylate, n-butyl methacrylate and the like, acrylates such as methyl acrylate, ethyl acrylate, n-butyl acrylate and the like, acrylic acid, methacrylic acid, maleic anhydride (also maleic acid), vinyl ethylene carbonate, and vinyl chloride. Although ethylene is not preferred as a comonomer for the high Tg polymer due to its effect in suppressing the dry basis Tg to below room temperature, it could be incorporated at low levels or utilized to balance the Tg if a higher Tg termonomer is incorporated into the structure.
The lower Tg copolymers of vinyl acetate are preferably copolymers of vinyl acetate and ethylene and copolymers of vinyl acetate and acrylic comonomers with the vinyl acetate making up at least 60 weight percent of the copolymer. The minimum amount of comonomer is that required to achieve the desired dry Tg for the copolymer. The acrylic monomer is an alkyl acrylate, preferably with the alkyl group containing 1 to 8 carbons, such as n-butyl acrylate, methyl acrylate, ethyl acrylate, 2-ethyl hexyl acrylate, and the like. Also included as possible copolymers of vinyl acetate are copolymers containing a third monomer, such as a terpolymer of vinyl acetate, ethylene and acrylates, such as n-butyl acrylate, ethyl acrylate, and the like. The copolymer of vinyl acetate and ethylene can also contain vinyl chloride as a third monomer. These are also preferred compositions. The vinyl acetate copolymers as described above can also contain small amounts of other monomers such as acrylic acid, methacrylic acid, maleic anhydride, and the like.
The vinyl acetate polymers of this invention can be produced by emulsion polymerization and the technology for polymerization is well known, having been commercially practiced for over four decades. These emulsions are prepared by the addition of vinyl acetate (and other monomers) to water containing a surfactant and/or protective colloid system. These surfactants can include anionic, non-ionic, and cationic surfactants. Generally anionic and non-ionic surfactants are preferred with non-ionic most preferred for this invention. The non-ionic surfactants include ethoxylated alkyl phenols and ethylene oxide/propylene oxide block copolymers. Protective colloids can also be employed as is or in admixtures with other surfactants. The preferred protective colloids are poly(vinyl alcohol) and hydroxyethylcellulose (HEC). The polymerization of the vinyl acetate homopolymers and copolymers of this invention involves free radical initiated polymerization. Typical free radical initiators include organic peroxides, azo initiators (such as 2,2xe2x80x2-azobisisobutyronitrile), peroxysulfates and redox initiation systems. Redox initiation systems include persulfate-bisulfate, hydrogen peroxide-iron, hydroperoxide-iron, and chlorate-bisulfate systems. Specific systems include a combination of potassium methyl acrylate, sodium or ammonium persulfate with various reducing agents such as sodium hydrogen sulfite, ascorbic acid, erythrobic acid, sodium formaldehyde sulfoxylate, and the like. Chain transfer agents such as mercaptans and thiols can be added to control the molecular weight. Additional details of these emulsion polymers and their preparation are detailed in discussions by Daniels in Encyclopedia of Polymer Science and Engineering, Volume 17, xe2x80x9cVinyl Ester Polymersxe2x80x9d, pp. 393-425, Wiley-lnterscience, New York, 1989, and by Vaandezande et al. in Emulsion Polymerization and Emulsion Polymers, edited by P. A. Lovell and M. S. El-Aasser, xe2x80x9cVinyl Acetate Polymerizationxe2x80x9d, pp. 563-584, John Wiley and Sons, New York, 1997.
The emulsion blends of this invention can be formulated to yield satin, semi-gloss or high gloss architectural coatings (paint) materials. The additives typically employed in such formulations include pigments such as TiO2, fillers including CaCO3, talc, mica, barium sulfate, silica and the like, clays, dispersing agents such as tetrasodium pyrophosphate, poly(acrylic acids) or soya lecithin, wetting agents, defoamers such as acetylenic diols, mineral oils or silicones, plasticizers, associative thickeners for rheology control, waxes, colorants, antioxidants, UV stabilizers, biocides, wet-adhesive emulsion additives, acrylic based emulsions, styrene based emulsions, coalescing agents such as texanol, butyl carbinol, hexylene glycol or ethylene glycol monobutyl ether, adipic, phthalic and benzoic acid esters of propane diol, propylene glycol ether, and the like, and additives for pH control. Compounds promoting freeze-thaw stability such as ethylene glycol and propylene glycol can also be added. A review of the technology involved with architectural coatings is given by Lowell in xe2x80x9cCoatingsxe2x80x9d, Encyclopedia of Polymer Science and Engineering, Vol. 3, pp. 615-675, Wiley-Interscience, New York, 1989. In general, formulations for glossy paints, including satin, semi-gloss and high gloss paints are well known in the art and do not require further elaboration.
The coatings of this invention can be applied by spray techniques, brushed onto substrates, applied with fiber-based rollers, applied using roll coating equipment and the like. The substrates to which the coatings of this invention can be applied include wood-based, plasterboard, cement, wallpaper, previously coated surfaces, stucco, leather, plastic-based surfaces, plastic film, paper, cardboard, metal and the like. The coatings are utilized preferably in interior applications but exterior applications can also be considered. The application to poly(vinyl chloride), PVC, (vinyl) based surfaces can be employed as the VAE (vinyl acetate-ethylene) compositions of this invention have demonstrated good adhesion to such substrates.
The levels of addition of the high Tg vinyl acetate polymers to the low Tg vinyl acetate copolymer emulsions according to this invention are in the range of 5 to 75 weight percent, solids basis, based on the total emulsion polymers used, including both the high Tg and the low Tg polymers plus any other emulsion polymer additions. Preferably the proportion of the high Tg vinyl acetate polymer is from 20 to 60 weight percent, and even more preferably is 20 to 50 weight percent.
The desired particle size for the higher Tg vinyl acetate polymer is an average diameter of less than 0.35 microns and preferably less than 0.25 microns. The desired particle size for the lower Tg vinyl acetate based emulsion is an average diameter of less than 0.5 microns with a preferred average particle size diameter of less than 0.35 microns. The emulsion polymer blends of this invention are novel for semi-gloss paints as well as high gloss or satin type coating systems in which advantages in maintaining gloss values are realized. The gloss values which this invention is able to maintain or realize through improvement as read with a gloss meter off of paint surfaces at 60xc2x0 are greater than 10 for satin paints and greater than 30 for semi-gloss paints using the standard ASTM method.